Surface treatment method for aluminum

ABSTRACT

The cleaned surface of an aluminum or aluminum alloy product, for example, aluminum coil, is treated with a 0.5 to 3 second spray of a chromating bath heated to 40° to 60° C., said bath having a pH of 1.0 to 3.0 and containing 0.1 to 3.0 g/L of tungstate ions and/or molybdate ions, 0.5 to 4.0 g/L of hexavalent chromium ions, 5.0 to 30.0 g/L of phosphate ions, and 0.05 to 2.0 g/L of free fluoride ions. A highly corrosion-resistant, highly paint-adherent chromate coating can thereby be formed on the surface of aluminum in less time than that required by prior phosphoric acid-based chromating treatments.

TECHNICAL FIELD

The present invention relates to a novel method for rapidly treating thesurface of aluminum and aluminum alloy (hereinafter jointly describedsimply as "aluminum", unless the context implies otherwise), a methodwhich rapidly produces on the aluminum a conversion film having achromium add-on equivalent to the chromium add-on of films produced byprior phosphoric acid-based chromating treatments and which provides thealuminum surface with an excellent corrosion resistance and paintadherence. The present invention is particularly well suited fortreating the surface of aluminum coil.

BACKGROUND ART

Conversion treatments which provide the surface of aluminum with anexcellent corrosion resistance and paint adherence are typified bychromic acid-based chromating treatments and phosphoric acid-basedchromating treatments. The conversion coatings produced by chromicacid-based chromating treatments (Japanese Patent Publication Number45-41088 [41,088/70] and British Patent Number 1,087,757) containhexavalent chromium and therefore have a yellow or gold color. Thus, thecolor of this film will remain visible when a clear coat is applied onthe surface. Moreover, since the conversion coating contains toxichexavalent chromium, the development of a Cr(VI)-free conversion coatingis desired.

In contrast to the preceding, the coatings formed by phosphoricacid-based chromating treatments do not contain hexavalent chromium andthus are colorless. As a result, the use of this treatment has beenspreading in recent years. The phosphoric acid-based chromatingtreatment disclosed in U.S. Pat. No. 2,438,877 employs a treatment bathof chromic acid (CrO₃), orthophosphoric acid (H₃ PO₄), and hydrofluoricacid (HF), and the principal component of the resulting film is chromicphosphate hydrate (CrPO₄ ·4H₂ O). When this particular phosphoricacid-based chromating treatment is run in an aluminum coil line usingthe older conversion treatment and times of 5 to 10 seconds, a stronglycorrosion-resistant, highly paint-adherent conversion coating having achromium add-on of 20 to 50 mg/m² is produced thereby.

Also within the sphere of Cr(VI)-free conversion treatment baths,Japanese Patent Application Laid Open [Kokai or Unexamined] Number63-86875 [86,875/88] teaches the addition of molybdate to a zincphosphate conversion treatment bath. However, the purpose of thisconversion treatment bath is to passivate the surface of compositematerials fabricated from different types of metals, and it alsocontains trivalent chromium, zinc oxide, and acetate as essentialcomponents in addition to molybdate.

DISCLOSURE OF THE INVENTION

On the subject of the aforementioned phosphoric acid-based chromatingtreatments, aluminum coil lines have been operated at increasing speedsin recent years in order to improve their productivity, and conversiontreatment times have been shortened to 1 to 3 seconds. However, a highlycorrosion-resistant, highly paint-adherent conversion film having achromium add-on ≧20 mg/m² essentially cannot be produced when priorphosphoric acid-based chromating treatments are installed in theseaccelerated lines, thus creating a problem which largely cannot beaddressed by prior phosphoric acid-based chromating treatments.Accordingly, the development of a phosphoric acid-based chromatingtreatment which can generate a conversion coating having a chromiumadd-on ≧20 mg/m² at conversion treatment times ≦3 seconds is desired.

The above-described problem was carefully examined, and it wasdiscovered as a result that a conversion film with an unusually goodcorrosion resistance and paint adherence can be formed on the surface ofaluminum by spraying same for 0.5 to 3 seconds with a chromating bath(heated to 40° to 60° C.) which has a pH of 1.0 to 3.0 and whichcontains 0.1 to 3.0 g/L of ions selected from tungstate ion and/ormolybdate ions, 0.5 to 4.0 g/L of hexavalent chromium ions, 5.0 to 30.0g/L of phosphate ions, and 0.05 to 2.0 g/L free fluoride ion. Thepresent invention was developed as a result of this discovery.

A source of tungstate (WO₄ ²⁻) ions source is exemplified by tungstatesalts such as, for example, ammonium tungstate, potassium tungstate,sodium tungstate, and lithium tungstate, and sodium tungstate isparticularly preferred. The concentration of these ions preferably fallswithin the range of 0.1 to 3.0 g/L for the following reasons: At lessthan 0.1 g/L, a satisfactory chromium add-on is not obtained even byspraying for 3 seconds, thereby precluding a good corrosion resistanceand paint adherence; at more than 3.0 g/L, the risk arises of theformation (and sedimentation) of salts formed with heavy metals such aschromium.

A source of molybdate (MoO₄ ²⁻) ions is exemplified by molybdate saltssuch as, for example, ammonium molybdate, potassium molybdate, sodiummolybdate, and lithium molybdate, and sodium molybdate is particularlypreferred. The concentration of these ions preferably falls within therange of 0.1 to 3.0 g/L for the following reasons: At less than 0.1 g/L,a satisfactory chromium add-on is not obtained even by spraying for 3seconds, thereby precluding a good corrosion resistance and paintadherence; at more than 3.0 g/L, the risk arises of the formation (andsedimentation) of salts formed with heavy metals such as chromium.

A satisfactory operation of the method according to the invention isalso obtained when the chromating bath employed contains both tungstateions and molybdate ions at a total concentration of 0.1 to 3.0 g/L.

Chromic acid (CrO₃) is preferably used to supply the hexavalent chromiumion. The concentration of these ions preferably falls within the rangeof 0.5 to 4.0 g/L for the following reasons: Less than 0.5 g/L preventsthe satisfactory development of the conversion film, resulting in adiminished corrosion resistance; exceeding 4.0 g/L makes the treatmentbath harder to treat for disposal and is therefore economically andenvironmentally disadvantageous.

Orthophosphoric acid (H₃ PO₄) is preferably employed to supply thephosphate ions. The concentration of these ions preferably falls withinthe range of 5.0 to 30.0 g/L for the following reasons: At below 5.0g/L, the resulting film contains only a small quantity of chromicphosphate and thus has a diminished paint adherence; a good film isformed with concentrations above 30.0 g/L, but the corresponding bathcost is so high as to be economically disadvantageous.

The free fluoride ions are the crucial component for controlling orinfluencing the growth rate of the conversion film. A source of freefluoride ions is exemplified by hydrofluoric acid, by fluorides such assodium fluoride and ammonium fluoride, and by complex fluorides such asthe fluosilicates and fluoborates. The free fluoride ion concentrationin the chromating bath was determined using an ion-selective electrode(the F-125 fluoride-selective electrode and HS-305DP reference electrodefrom Toa Denpa Kogyo Kabushiki Kaisha) and an ion meter (the IM40S fromToa Denpa Kogyo Kabushiki Kaisha). The ion meter was calibrated using areference solution prepared by the addition of a known quantity ofhydrofluoric acid (for example, 0.1 g/L, 1 g/L, or 10 g/L) to 5 g/Lchromic acid and 15 g/L orthophosphoric acid and adjusting the pH to 2.0with orthophosphoric acid or sodium hydroxide. The free fluoride ionconcentration in this reference solution corresponded to the totalquantity of fluorine in the hydrofluoric acid added. Then, afteradjusting the pH of the chromating bath to 2.0 with orthophosphoric acidor sodium hydroxide, it was measured using the fluorine ion meter, andthe obtained measured value was taken as the free fluoride ionconcentration. The free fluoride ion concentration preferably fallswithin the range of 0.05 to 2.0 g/L and more preferably within the rangeof 0.1 to 2.0 g/L. At below 0.05 g/L, the conversion coating grows tooslowly, which causes a low productivity since a lengthy treatment isthen required to generate a satisfactory coating. At above 2.0 g/L,severe aluminum etching occurs and a coating is not formed.

The pH of the chromating bath must be adjusted to within the range of1.0 to 3.0, and sodium hydroxide or ammonium hydroxide or an acid suchas orthophosphoric acid, nitric acid, hydrochloric acid, and the like ispreferably used as appropriate for this pH adjustment. At below pH 1.0,severe etching occurs and film formation is hindered. When the pHexceeds 3.0, etching is weak and a uniform coating cannot usually beformed.

The surface treatment method according to the present invention can beused as a substitute for the phosphoric acid-based chromating treatmentwhich is in wide use at the present time. The surface of the aluminumshould normally first be cleaned when executing a chromating treatmenton the surface of aluminum using the surface treatment method accordingto the present invention. The cleaning method here encompasses treatmentwith an acidic, alkaline, or solvent-based cleaning solution or acombination thereof.

The cleaned aluminum surface can also be subsequently etched with acidor alkali as desired. Treatment with the chromating bath employed by thepresent invention is conducted by spraying, and the treatmenttemperature at this point should fall within the range of 40° to 60° C.Spraying this chromating bath (held in the aforementioned temperaturerange) on a clean surface of aluminum for 0.5 to 3 seconds will formthereon a chromic phosphate hydrate (CrPO₄ ·4H₂ O) coating having achromium add-on of at least 20 mg/m².

The chromated aluminum surface is then normally rinsed with water andfinally drained and dried. These water rinse and drying steps are notnarrowly restricted within the context of the present invention, andthey may be implemented using means known in the art. However, the drainand dry step is preferably conducted at an aluminum sheet temperature ofapproximately 60° to 80° C. The chromium add-on preferably falls withinthe range of 20 to 40 milligrams per square meter (hereinafterabbreviated as "mg/m² ") of the surface treated. Based on aconsideration of the required corrosion resistance, the chromium add-oncan be controlled by appropriately adjusting the bath concentration andtemperature and the spray time.

EXAMPLES

The present invention is explained below using several illustrativeexamples, whose utility is compared with comparison examples.

JIS 6063 aluminum alloy was used as the substrate. A small sprayer wasused for degreasing and chromating this aluminum alloy, and this smallsprayer was designed to give the same spray conditions as on apresent-day continuous spray treatment line for chromating aluminumcoil.

The corrosion resistance was evaluated using salt spray testing and hotwater immersion testing. Salt spray testing was conducted in accordancewith JIS Z 2371, and the extent of corrosion development on theunpainted chromated sheet was evaluated using rating numbers afterspraying for 150 hours. In this system, higher rating numbers indicate abetter corrosion resistance, and a rating number at or above 9.0 isdeemed satisfactory. The hot water immersion test was run by immersingthe chromated sheet in hot water for 30 minutes and then visuallyinspecting for changes in surface color. A five-level scale was used forevaluation in the hot water immersion test, with scores ranging from 1(poor) to 5 (excellent).

In order to evaluate the paint adherence, a commercial black sprayacrylic lacquer was applied to a film thickness of 5 to 10 micrometerson the chromated aluminum alloy sheet and baked at 200° C. for 10minutes to afford the test sheet. A 4.5 mm protrusion was executed inthe painted test sheet according to Erichsen test method A in JIS Z2247, and paint film peeling on the protruded region was visuallyevaluated after peeling with tape (primary adhesion). In order toevaluate the water resistance of the adhesion, the painted sheet wasimmersed in hot water for 30 minutes, a 4.5 mm protrusion was thenexecuted in the painted test sheet according to Erichsen test method Ain JIS Z 2247, and paint film peeling on the protruded region wasvisually evaluated after peeling with tape (secondary adhesion). Afive-level scale was used for evaluation of the paint adherence, withscores ranging from 1 (poor) to 5 (excellent).

The chromium add-on in the chromate film was determined using an X-rayfluorescence analyzer (model 3070E from Kabushiki Kaisha Rigaku).

EXAMPLE 1

The surface of the aluminum alloy was cleaned by rinsing for 2 secondswith a hot (70° C.) 2% aqueous solution of a commercial alkalinedegreaser (FINECLEANER™ 4377A, registered brand name of NihonParkerizing Company, Limited) followed by rinsing with water. This wasfollowed by a 2 second spray with chromating bath 1 heated to 50° C.,rinsing with water, spraying for 2 seconds with deionized water(≧3,000,000 ohm-cm), and finally drying with hot air at 70° C. for 1minute. After drying, the corrosion resistance and paint adherence wereevaluated under the conditions described above.

EXAMPLE 2

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 2 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 1                                                         ______________________________________                                        Composition of Chromating Bath 1                                              Component           Content of Component                                      ______________________________________                                        Sodium tungstate (Na.sub.2 WO.sub.4.2H.sub.2 O)                                                    2.5 g/L (WO.sub.4.sup.2-  1.9 g/L)                       Chromic acid (CrO.sub.3)                                                                           5.0 g/L (Cr.sup.6+  2.6 g/L)                             75% phosphoric acid (H.sub.3 PO.sub.4)                                                            30.0 g/L (PO.sub.4.sup.3-  21.8 g/L)                      20% hydrofluoric acid (HF)                                                                         8.5 g/L (free F.sup.-  1.6 g/L)                          pH: 2.0 (adjusted with sodium hydroxide)                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Composition of Chromating Bath 2                                              Component           Content of Component                                      ______________________________________                                        Sodium tungstate (Na.sub.2 WO.sub.4.2H.sub.2 O)                                                    0.5 g/L (WO.sub.4.sup.2-  0.4 g/L)                       Chromic acid (CrO.sub.3)                                                                           3.5 g/L (Cr.sup.6+  1.8 g/L)                             75% phosphoric acid (H.sub.3 PO.sub.4)                                                            16.8 g/L (PO.sub.4.sup.3-  12.2 g/L)                      20% hydrofluoric acid (HF)                                                                         6.5 g/L (free F.sup.-  1.2 g/L)                          pH: 2.0 (adjusted with sodium hydroxide)                                      ______________________________________                                    

EXAMPLE 3

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 3 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 3                                                         ______________________________________                                        Composition of Chromating Bath 3                                              Component           Content of Component                                      ______________________________________                                        Sodium tungstate (Na.sub.2 WO.sub.4.2H.sub.2 O)                                                   1.0 g/L (WO.sub.4.sup.2-  0.8 g/L)                        Chromic acid (CrO.sub.3)                                                                          1.6 g/L (Cr.sup.6+  0.8 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                            9.0 g/L (PO.sub.4.sup.3-  6.5 g/L)                        Ammonium fluoride (NH.sub.4 F)                                                                    1.5 g/L (free F.sup.-  0.7 g/L)                           pH: 1.4 (adjusted with orthophosphoric acid)                                  ______________________________________                                    

EXAMPLE 4

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 4 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 4                                                         ______________________________________                                        Composition of Chromating Bath 4                                              Component          Content of Component                                       ______________________________________                                        Sodium molybdate    3.6 g/L (MoO.sub.4.sup.2-  2.4 g/L)                       (Na.sub.2 MoO.sub.4.2H.sub.2 O)                                               Chromic acid (CrO.sub.3)                                                                          5.0 g/L (Cr.sup.6+  2.6 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                           30.0 g/L (PO.sub.4.sup.3-  21.8 g/L)                       Sodium fluoride (NaF)                                                                             3.0 g/L (free F.sup.-  1.3 g/L)                           pH: 2.0 (adjusted with sodium hydroxide)                                      ______________________________________                                    

EXAMPLE 5

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 5 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 5                                                         ______________________________________                                        Composition of Chromating Bath 5                                              Component          Content of Component                                       ______________________________________                                        Sodium molybdate    0.5 g/L (MoO.sub.4.sup.2-  0.3 g/L)                       (Na.sub.2 MoO.sub.4.2H.sub.2 O)                                               Chromic acid (CrO.sub.3)                                                                          3.5 g/L (Cr.sup.6+  1.6 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                           16.8 g/L (PO.sub.4.sup.3-  12.2 g/L)                       20% hydrofluoric acid (HF)                                                                        6.5 g/L (free F.sup.-  total                              40% fluosilicic acid (H.sub.2 SiF.sub.6)                                                          3.0 g/L of 1.3 g/L)                                       pH: 2.0 (adjusted with sodium hydroxide)                                      ______________________________________                                    

EXAMPLE 6

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 6 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 6                                                         ______________________________________                                        Composition of Chromating Bath 6                                              Component           Content of Component                                      ______________________________________                                        Sodium molybdate (Na.sub.2 MoO.sub.4.2H.sub.2 O)                                                  1.2 g/L (MoO.sub.4.sup.2-  0.8 g/L)                       Chromic acid (CrO.sub.3)                                                                          1.6 g/L (Cr.sup.6+  0.8 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                            9.0 g/L (PO.sub.4.sup.3-  6.5 g/L)                        20% hydrofluoric acid (HF)                                                                        4.0 g/L (free F.sup.-  0.7 g/L)                           pH: 1.4 (adjusted with orthophosphoric acid)                                  ______________________________________                                    

EXAMPLE 7

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 7 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 7                                                         ______________________________________                                        Composition of Chromating Bath 7                                              Component          Content of Component                                       ______________________________________                                        Sodium tungstate (Na.sub.2 WO.sub.4.2H.sub.2 O)                                                   1.0 g/L (WO.sub.4.sup.2-  0.8 g/L)                        Sodium molybdate    0.9 g/L (MoO.sub.4.sup.2- 0.6 g/L)                        (Na.sub.2 MoO.sub.4.2H.sub.2 O)                                               Chromic acid (CrO.sub.3)                                                                          6.0 g/L (Cr.sup.6+  3.1 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                           25.0 g/L (PO.sub.4.sup.3-  18.2 g/L)                       20% hydrofluoric acid (HF)                                                                        6.0 g/L (free F.sup.-  1.1 g/L)                           pH: 1.4 (adjusted with orthophosphoric acid)                                  ______________________________________                                    

EXAMPLE 8

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 1 second with chromating bath 1 heated to 60°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

EXAMPLE 9

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 1 second with chromating bath 4 heated to 60°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

Comparison Example 1

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 8 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 8                                                         ______________________________________                                        Composition of Chromating Bath 8                                              Component          Content of Component                                       ______________________________________                                        Sodium tungstate(Na.sub.2 WO.sub.4.2H.sub.2 O)                                                   0.05 g/L (WO.sub.4.sup.2-  0.04 g/L)                       Chromic acid (CrO.sub.3)                                                                          4.5 g/L (Cr.sup.6+  2.3 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                           16.8 g/L (PO.sub.4.sup.3- 12.2 g/L)                        Ammonium fluoride (NH.sub.4 F)                                                                    2.5 g/L (free F.sup.-  1.2 g/L)                           pH: 2.0 (adjusted with sodium hydroxide)                                      ______________________________________                                    

Comparison Example 2

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 9 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 9                                                         ______________________________________                                        Composition of Chromating Bath 9                                              Component          Content of Component                                       ______________________________________                                        Sodium molybdate   0.06 g/L (MoO.sub.4.sup.2-  0.04 g/L)                      (Na.sub.2 MoO.sub.4.2H.sub.2 O)                                               Chromic acid (CrO.sub.3)                                                                          4.5 g/L (Cr.sup.6+  2.3 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                           16.8 g/L (PO.sub.4.sup.3-  12.2 g/L)                       20% hydrofluoric acid (HF)                                                                        6.5 g/L (free.sup.-  1.2 g/L)                             pH: 2.0 (adjusted with sodium hydroxide)                                      ______________________________________                                    

Comparison Example 3

The aluminum alloy was cleaned under the same conditions as in Example 1and was then sprayed for 2 seconds with chromating bath 10 heated to 50°C. After treatment, washing and drying were carried out under the sameconditions as in Example 1, and performance evaluation was conducted asabove.

                  TABLE 10                                                        ______________________________________                                        Composition of Chromating Bath 10                                             Component          Content of Component                                       ______________________________________                                        Sodium tungstate (Na.sub.2 WO.sub.4.2H.sub.2 O)                                                  0.05 g/L (WO.sub.4.sup.2-  0.04 g/L)                       Sodium molybdate   0.05 g/L (MoO.sub.4.sup.2-  0.03 g/L)                      (Na.sub.2 MoO.sub.4.2H.sub.2 O)                                               Chromic acid (CrO.sub.3)                                                                          5.0 g/L (Cr.sup.6+  2.6 g/L)                              75% phosphoric acid (H.sub.3 PO.sub.4)                                                           19.0 g/L (PO.sub.4.sup.3-  13.8 g/L)                       20% hydrofluoric acid (HF)                                                                        6.0 g/L (free F.sup.-  1.1 g/L)                           pH: 1.4 (adjusted with orthophosphoric acid)                                  ______________________________________                                    

Comparison Example 4

The aluminum alloy was cleaned under the same conditions as in Example 1and was sprayed for 2 seconds with a 4% aqueous solution of a commercialphosphoric acid-based chromating agent (ALCHROM™ K702, registered brandname of Nihon Parkerizing Company, Limited), heated to 50° C. Aftertreatment, washing and drying were carried out under the same conditionsas in Example 1, and performance evaluation was conducted as above.

The results of these examples and comparison examples, as reported inTable 11, confirm that the present invention produces an excellentcorrosion resistance and excellent paint adherence.

                  TABLE 11                                                        ______________________________________                                        Results of Evaluation Testing                                                              Corrosion Resist-                                                             ance Tests                                                                               Hot     Paint Adherence                                      Chromium                                                                              SST, 150 Water   Tests                                                Add-on, Hours,   Immer-  Pri- Second-                                         mg/m.sup.2                                                                            Rating   sion    mary ary                                      ______________________________________                                        Example 1                                                                              29        9.8      5     5    4                                      Example 2                                                                              22        9.5      5     5    4                                      Example 3                                                                              20        9.2      5     5    4                                      Example 4                                                                              28        9.7      5     5    4                                      Example 5                                                                              21        9.3      5     5    4                                      Example 6                                                                              20        9.0      5     5    4                                      Example 7                                                                              29        9.8      5     5    4                                      Example 8                                                                              22        9.2      5     5    4                                      Example 9                                                                              22        9.2      5     5    4                                      Comparison                                                                             15        7.0      4     4    3                                      Example 1                                                                     Comparison                                                                             15        7.0      4     4    3                                      Example 2                                                                     Comparison                                                                             15        7.0      4     4    3                                      Example 3                                                                     Comparison                                                                             13        6.0      3     3    2                                      Example 4                                                                     ______________________________________                                    

Benefits of the Invention

As explained hereinbefore, the present invention provides a highlycorrosion-resistant, highly paint-adherent chromate coating on thesurface of aluminum in less time than that required for prior phosphoricacid-based chromate treatments. This is achieved by spraying an acidicchromating bath that contains free fluoride ion, phosphate ion,hexavalent chromium ion, and tungstate ion and/or molybdate ion.

The invention claimed is:
 1. A method for treating an aluminum surfaceto provide thereon a corrosion protective and paint adherence promotinglayer by spraying the surface for 0.5 to not more than 3 seconds with achromating solution at a temperature of 40° to 60° C. and a pH of 1.0 to3.0, said chromating solution consisting essentially of:(A) from 0.1 to3.0 g/L of ions selected from the group consisting of tungstate ions,molybdate ions and both tungstate and molybdate ions; (B) from 0.5 to4.0 g/L, measured as hexavalent chromium, of ions containing hexavalentchromium; (C) from 5.0 to 30.0 g/L of phosphate ions; and (D) from 0.05to 2.0 g/L of free fluoride ions wherein the areal density of chromiumin said corrosion protective and paint adherence promoting layer is atleast 20 mg/m².
 2. A method according to claim 1, wherein the chromatingsolution contains from 0.1 to 2.0 g/L of free fluoride ions.
 3. A methodaccording to claim 2, wherein the areal density of chromium in thecorrosion protective and paint adherence promoting layer formed duringthe process is not more than 40 mg/m².
 4. A method according to claim 3,wherein, in the chromating solution, the content of component (A) isderived from dissolved sodium tungstate, sodium molybdate, or a mixturethereof; the content of hexavalent chromium is derived from dissolvedchromic acid; the content of phosphate ions is derived from dissolvedorthophosphoric acid; and the content of free fluoride is derived fromat least one of hydrofluoric acid, sodium fluoride, ammonium fluoride,and fluorosilicic acid.
 5. A method according to claim 4, wherein thealuminum surface contacted is that of aluminum coil.
 6. A methodaccording to claim 5, comprising additional steps of rinsing thealuminum surface after chromating and drying the rinsed surface at analuminum temperature of 60°-80° C.
 7. A method according to claim 6,wherein the aluminum surface contacted is that of aluminum coil, and themethod additionally comprises the steps of rinsing the aluminum surfaceand subsequently drying at an aluminum temperature of 60°-80° C.
 8. Amethod according to claim 4, wherein the aluminum surface contacted isthat of aluminum coil, and the method additionally comprises the stepsof rinsing the aluminum surface and subsequently drying at an aluminumtemperature of 60°-80° C.
 9. A method according to claim 3, wherein thealuminum surface contacted is that of aluminum coil, and the methodadditionally comprises the steps of rinsing the aluminum surface andsubsequently drying at an aluminum temperature of 60°-80° C.
 10. Amethod according to claim 2, wherein the aluminum surface contacted isthat of aluminum coil, and the method includes additional steps ofrinsing the aluminum surface after chromating and drying the rinsedsurface at an aluminum temperature of 60°-80° C.
 11. A method accordingto claim 2, wherein the aluminum surface contacted is that of aluminumcoil, and the method additionally comprises the steps of rinsing thealuminum surface and subsequently drying at an aluminum temperature of60°-80° C.
 12. A method according to claim 1, wherein the areal densityof chromium in the corrosion protective and paint adherence promotinglayer formed during the process is not more than 40 mg/m².
 13. A methodaccording to claim 12, wherein the aluminum surface contacted is that ofaluminum coil, and the method additionally comprises the steps ofrinsing the aluminum surface and subsequently drying at an aluminumtemperature of 60°-80° C.
 14. A method according to claim 1, wherein, inthe chromating solution, the content of component (A) is derived fromdissolved sodium tungstate, sodium molybdate, or a mixture thereof; thecontent of hexavalent chromium is derived from dissolved chromic acid;the content of phosphate ions is derived from dissolved orthophosphoricacid; and the content of free fluoride is derived from at least one ofhydrofluoric acid, sodium fluoride, ammonium fluoride, and fluorosilicicacid.
 15. A method according to claim 14 wherein the aluminum surfacecontacted in that of aluminum coil, and the method additionallycomprises the steps of rinsing the aluminum surface and subsequentlydrying at an aluminum temperature of 60°-80° C.
 16. A method accordingto claim 1, wherein the aluminum surface contacted is that of aluminumcoil, and the method includes additional steps of rinsing the aluminumsurface after chromating and drying the rinsed surface at an aluminumtemperature of 60°-80° C.
 17. A method according to claim 1, wherein thealuminum surface contacted is that of aluminum coil, and the methodadditionally comprises the steps of rinsing the aluminum surface andsubsequently drying at an aluminum temperature of 60°-80° C.